The goals of this research are (a) to describe the early development of empathic, prosocial behaviors and aggression, (b) to identify biological and environmental factors that contribute to individual differences and (c) ultimately to examine the contributions of these factors to later patterns of adjustment and maladjustment in development. A behavior genetics, longitudinal research design is used to compare patterns of concordance of empathy and aggression in 350 pairs of monozygotic and dizygotic twins. Cognitive, affective and behavioral components of empathy are coded. Current heritability analyses are based on 100 pars of mz twins and 100 pairs of dz twins observed both in the home and laboratory at 14 and 20 months of age. Analyses focus only on empathic behaviors as these emerge earlier in development than aggression. Preliminary analyses indicate that certain components of empathy show significant (low-to-moderate) stability over time. There is evidence for heritability of empathy, particularly in young females who show higher levels of responsiveness to others' distress, and particularly for the affective components. The evidence for heritability, while statistically significant, is not strong and hence suggests that environmental forces will play a substantial role in shaping children's capacities to show concern for others. This conclusion is in contrast with the generalizations from other self-report studies of adult twins, in which the bulk of variance in empathy/altruism is ascribed to genetic influence.